For measuring a target point, numerous geodetic surveying devices have been known since ancient times. In this case, direction or angle and usually also distance from a measuring device to the target point to be measured are recorded and, in particular, the absolute position of the measuring device together with reference points possibly present are detected as spatial standard data.
Generally known examples of such geodetic surveying devices include the theodolite, tachymeter and total station, which is also designated as electronic tachymeter or computer tachymeter. One geodetic measuring device from the prior art is described in the publication document EP 1 686 350, for example. Such devices have electrical-sensor-based angle and, if appropriate, distance measuring functions that permit direction and distance to be determined with respect to a selected target. In this case, the angle and distance variables are determined in the internal reference system of the device and, if appropriate, also have to be combined with an external reference system for absolute position determination.
In many geodetic applications, points are measured by specifically configured target objects being positioned there. The latter usually consist of a plumb staff with a reflector (e.g. an all-round prism) for defining the measurement path or the measurement point. In the case of such measurement tasks, for controlling the measurement process and for defining or registering measurement parameters, a number of data, instructions, speech and further information are transmitted between target object—in particular a handheld data acquisition device at the target object—and central measuring device. Examples of such data include the identification of the target object (type of prism used), inclination of the plumb staff, height of the reflector above ground, reflector constants or measurement values such as temperature or air pressure. These information items or situation-governed parameters are necessary for enabling highly precise targeting and measurement of the measurement point defined by the plumb rod with prism.
Modern total stations have microprocessors for digital further processing and storage of detected measurement data. The devices generally have a compact and integrated design, wherein coaxial distance-measuring elements and also computing, control and storage units are usually present in a device. Depending on the expansion stage of the total station, motorization of the targeting or sighting device and—in the case of the use of retroreflectors (for instance an all-round prism) as target objects—means for automatic target seeking and tracking can additionally be integrated. As a human-machine interface, the total station can have an electronic display control unit—generally a microprocessor computing unit with electronic data storage means—with display and input means, e.g. a keyboard. The measurement data detected in an electrical-sensor-based manner are fed to the display control unit, such that the position of the target point can be determined, optically displayed and stored by the display control unit. Total stations known from the prior art can furthermore have a radio data interface for setting up a radio link to external peripheral components such as e.g. a handheld data acquisition device, which can be designed, in particular, as a data logger or field computer.
For sighting or targeting the target point to be measured, geodetic surveying devices of the generic type have a telescopic sight, such as e.g. an optical telescopic sight, as sighting device. The telescopic sight is generally rotatable about a vertical axis and about a horizontal tilt axis relative to a base of the measuring device, such that the telescopic sight can be aligned with the point to be measured by pivoting and tilting. Modern devices can have, in addition to the optical viewing channel, a camera for detecting an image, said camera being integrated into the telescopic sight and being aligned for example coaxially or in a parallel fashion, wherein the detected image can be represented, in particular, as a live image on the display of the display control unit and/or on a display of the peripheral device—such as e.g. the data logger—used for remote control. In this case, the optical system of the sighting device can have a manual focus—for example an adjusting screw for altering the position of a focusing optical system—or an autofocus, wherein the focus position is altered e.g. by servomotors. By way of example, such a sighting device of a geodetic surveying device is described in EP 2 219 011. Automatic focusing devices for telescopic sights of geodetic devices are known e.g. from DE 197 107 22, DE 199 267 06 or DE 199 495 80.
The optical system or the optical viewing channel of the sighting device usually contains an objective lens group, an image reversal system, a focusing optical system, a graticule for producing a reticle and an eyepiece, which are arranged e.g. in this order from the object side. The position of the focusing lens group is set depending on the object distance in such a way that a sharp object image arises on the graticule arranged in the focusing plane. Said image can then be viewed through the eyepiece or e.g. detected with the aid of a camera arranged coaxially.
By way of example, the construction of generic telescopic sights of geodetic devices is disclosed in the publication documents EP 1 081 459 or EP 1 662 278.
On account of the beam path that is usually to be utilized jointly both as viewing channel and for measurements, such devices require the technical design of said beam path in the manner of construction of a telescopic sight with specialized, high-precision optical systems that are to be produced with a high outlay. Furthermore, an additional separate transmitting and receiving channel and also an additional image plane for the wavelength of the distance measuring device are provided for the coaxial electronic distance measurement.
Since target objects (e.g. the plumb rods with target mark, such as an all-round prism, which are usually used for geodetic purposes) can be targeted sufficiently precisely with the naked eye on the basis of the sighting device despite the 30-fold optical magnification often provided (i.e. not conforming to geodetic accuracy requirements), conventional surveying devices in the meantime have as standard an automatic target-tracking function for prisms serving as target reflector (ATR: “Automatic Target Recognition”). For this, a further separate ATR light source—e.g. a multimode fiber output, which emits optical radiation having a wavelength in the range of 850 nm—and a specific ATR detector (e.g. CCD area sensor) sensitive to said wavelength are conventionally additionally integrated in the telescopic sight.
In the context of the ATR fine-targeting function, in this case the ATR measurement beam is emitted in the direction of the optical targeting axis of the sighting device and is retroreflected at the prism and the reflected beam is detected by the ATR sensor. Depending on the deviation of the alignment of the optical targeting axis from the prism, in this case the impingement position of the reflected radiation on the ATR sensor also deviates from a central sensor area position (i.e. the reflection spot of the ATR measurement beam retroreflected at the prism on the ATR area sensor does not lie in the center of the ATR area sensor and therefore does not impinge on a desired position defined e.g. on the basis of calibration as that position which corresponds to the optical targeting axis).
If this is the case, if the alignment of the sighting device is slightly readjusted in a motor-driven manner in such a way that the ATR measurement beam retroreflected at the prism impinges highly precisely in the center of the sensor area on the ATR area sensor (i.e. the horizontal and vertical angles of the sighting device are thus iteratively changed and adapted until the center of the reflection spot coincides with the desired position on the ATR area sensor). Alternatively, a residual deviation between the impingement point of the retroreflected ATR measurement beam on the ATR area sensor and the center of the sensor area can also be taken into account computationally and converted into an angle, which is correspondingly added to the solid angle—detected with the aid of the angle sensors—at which the targeting axis points. In other words, the solid angle with respect to the target point can in this case also be derived from the solid angle—detected with the aid of the angle sensors—of the targeting axis and an offset of the detected ATR measurement beam reflection from the sensor center (i.e. from that central point on the ATR sensor at which the targeting axis is imaged).
As a result, the achievable accuracy in the alignment of the optical targeting axis with the prism can be significantly increased by comparison with manually performed targeting with a reticle and on the basis of measurement by the naked eye. In order to ensure the functioning of the automatic targeting on the basis of evaluation of the position of the reflection spot of the ATR measurement beam retroreflected at the prism on the ATR area sensor, it is necessary, before the function starts, to align the sighting device with the target reflector at least approximately in such a way that the retroreflected ATR measurement beam also impinges on the ATR area sensor. For this purpose, it is possible e.g. beforehand to effect manual targeting of the target reflector on the basis of measurement by eye or to perform an automatic coarse targeting function.
The manual, coarse sighting of the target object can be effected by the user, on the one hand, by viewing and targeting the target object on a user display of the display control unit directly on the surveying device or on the display of a separate peripheral device (e.g. data logger as remote control). Often, however, this is still effected by means of viewing the target through the eyepiece of the telescopic sight (i.e. of the sighting device), since a displayed display image on the display control unit or the data logger may be insufficiently recognizable during use in practice—e.g. in the case of insolation.
Furthermore, the prior art discloses methods wherein the solid angles between the measuring device and the target object are determined with the aid of an image and image processing (in particular on the basis of a position of a target object determined in the recorded image and depending on a known or also detected image-recording direction). Such methods are described e.g. in WO 2005/026767 or in WO 2006/053837.
There is a disadvantage both in the case of a manual and in the case of an image-based automatic fine alignment of a surveying device on a target due to the typical design of the targeting unit with a focusing lens group, the design of which must be set very precisely and robustly and the position of which must be set depending on a given distance to a target to be targeted in order to obtain a clear and in-focus image of the target without aberration.
For the purposes of accurate and reliable targeting of the target, it is necessary in this case to focus accurately on the object which is actually targeted. By way of example, if an obstacle (e.g. a branch of a tree) is situated directly along the measurement axis, with this obstacle being placed at a significantly different distance than the target, and if the target is depicted in focus for targeting purposes, what may occur is that a distance to the obstacle but not to the target is determined. In such cases, the obstacle can no longer be perceived in the case of corresponding defocusing—for example by the user of the system.
In the case where the user notices that the determined distance cannot correspond to an actual distance to the target, an exact alignment and renewed measurement is connected with an additional, significantly increased time outlay, which has a negative effect on the productivity. In this case, a distance measurement according to the prior art to non-cooperative and far-away targets can take several seconds.
By way of example, focusing of an object to be targeted can be brought about by means of a contrast measurement. Here, the focusing member is displaced over the whole focusing range and, simultaneously, an image sequence of the object is recorded at short time intervals by means of the integrated camera. The image processing is used to establish the image from this image sequence in which the object is imaged with the highest contrast, and the focusing member is moved to the corresponding position. A disadvantage of this method once again includes the time required for moving the focusing member over the whole range and the image evaluation only being carried out subsequently.
In this context, a further disadvantage emerges from the structural design of a displacement unit for the focusing member. In particular, the optical axis of the main objective lens in a telescopic sight coincides with the optical axis of the focusing member. However, deviations which emerge from limitations in the mechanical implementation lead to aberrations in the optical image, which can have a negative influence on the targeting accuracy. On the one hand, such deviations emerge from the mechanical guide of the focusing member not being parallel with the optical axis of the objective lens, on the other hand, a certain amount of mechanical play on this guide must be accepted for a fast movement of the focusing member.